Bilateral electrolytic lesions in the suprachiasmatic nuclei permanently eliminated nocturnal and circadian rhythms in drinking behavior and locomotor activity of albino rats. The generation of 24-hr behavioral rhythms and the entrainment of these rhythms to the light-dark cycle of environmental illumination may be coordinated by neurons in the suprachiasmatic region of the rat brain. Destruction of the medial preoptic area had no effect on 24-hr drinking rhythms.The widespread occurrence and biological significance of 24-hr rhythms has been extensively documented (1, 2). Although rhythmic variations in animal behavior and physiology are ordinarily synchronized with fluctuations in light and temperature (1), many rhythms persist in free-running form with periods of about 24 hr (circadian rhythms) in the absence of all obvious entraining stimuli. Such demonstrations are generally interpreted as reflecting the operation of a biological clock within the animal (1).The identification of the neural substrate responsible for the 24-hr behavioral rhythms of mammals has yet to be accomplished (3,4). Circadian rhythms have been remarkably resistant to many forms of interference with the nervous system, including such radical treatments as anoxia, convulsions, poisoning, anesthesia, and acute stress (4). Some loss of rhythmicity in eating, but not in drinking, behavior of rats occurs after lesions are placed in the region of the ventromedial hypothalamus (5). However, these lesions also interfere with the homeostatic control of eating and body weight, and it is difficult to assess their effects on the biological clock per se.Several circadian neuroendocrine rhythms have been eliminated by lesions or surgical isolation of the anterior hypothalamus from the medial basal hypothalamus (6). In addition, recent anatomical studies have once again raised the possibility of direct visual input to the anterior hypothalamus via retino-hypothalamic pathways terminating in the suprachiasmatic nuclei and arcuate region (7,8). These considerations, and our previous failure to disrupt nocturnal drinking rhythms with lesions that interrupted the primary and accessory visual pathways (9), suggested to us that the suprachiasmatic region might be involved in the generation and entrainment of behavioral rhythms.In the present experiment we attempted to interfere with circadian drinking and activity rhythms of rats by selectively damaging several regions of the hypothalamus.Abbreviations: LD, light-dark cycle; SCN, suprachiasmatic nucleus; MPO, medial preoptic nucleus. METHODSAdult ovariectomized Sprague-Dawley rats were housed individually in cages with free access to food and water. The experimental room was illuminated by fluorescent lights providing cool white light; the average intensity of illumination at the face of the cage was 6 ft-c. The light-dark cycle (L-D) consisted of alternating 12-hr periods of light and darkness; the dark period began at 9 p.m. At various times animals were maintained on a reversed I-D cycle (the dail...

It is not surprising that limiting food access to a particular time of day has profound effects on the behavior and physiology of animals. It has been clear for some time that pre-meal behavioral activation, a rise in core temperature, elevated serum corticosterone, and an increase in duodenal disaccharidases are under circadian control and that the observed circadian properties are not abolished by lesions of the suprachiasmatic nucleus (SCN), but the search for the locus of a separate food-entrainable oscillator (FEO) has not been successful. The cloning of circadian clock genes and the discovery that these genes are expressed in many central nervous system structures outside the SCN and in peripheral tissues have led to new strategies for investigating potential loci of an FEO. Recent findings concerning the entrainment of clock gene expression in the central nervous system and in peripheral tissues by periodic food access are presented, and the implications of these findings for a better understanding of a circadian system that entrains to meals, rather than to light, are discussed.

Although the role of nucleus accumbens (NAcc) dopamine (DA) in reward learning has been extensively studied, few investigations have addressed its involvement in learning socially relevant information. Here, we have examined the involvement of NAcc DA in social attachment of the "monogamous" prairie vole (Microtus orchrogaster). We first demonstrated that DA is necessary for the formation of social attachment in male prairie voles, because administration of haloperidol blocked, whereas apomorphine induced, partner-preference formation. We then provided the first descriptions of DA neuroanatomy and tissue content in vole NAcc, and mating appeared to induce a 33% increase in DA turnover. We also showed that administration of haloperidol directly into the NAcc blocked partner preferences induced by mating and apomorphine. In addition, administration of apomorphine into the NAcc but not the caudate putamen induced partner preferences in the absence of mating. Together, our data support the hypothesis that NAcc DA is critical for pair-bond formation in male prairie voles.

It is not surprising that limiting food access to a particular time of day has profound effects on the behavior and physiology of animals. It has been clear for some time that pre-meal behavioral activation, a rise in core temperature, elevated serum corticosterone, and an increase in duodenal disaccharidases are under circadian control and that the observed circadian properties are not abolished by lesions of the suprachiasmatic nucleus (SCN), but the search for the locus of a separate food-entrainable oscillator (FEO) has not been successful. The cloning of circadian clock genes and the discovery that these genes are expressed in many central nervous system structures outside the SCN and in peripheral tissues have led to new strategies for investigating potential loci of an FEO. Recent findings concerning the entrainment of clock gene expression in the central nervous system and in peripheral tissues by periodic food access are presented, and the implications of these findings for a better understanding of a circadian system that entrains to meals, rather than to light, are discussed.

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